As a device for quickly determining a component of trace material included in a sample, a small and lightweight mass spectrometric device (often referred to as MS) has become necessary. In particular, a market is expanding as a sensing device of an illegal drug and an explosive. The mass spectrometric device ionizes a molecule in the sample to be analyzed, and detects an ion ionized by mass separation using an electric field and a magnetic field with a detector.
As a method for ionizing the molecule in the sample, APCI (Atmospheric Pressure Chemical Ionization Source), an electron impact ionization method, glow discharge, and the like have been put into practice; however, there are many inadequate points such as low ionization efficiency, and occurrence of fragmentation. Therefore, high precision adjustment is required to cope with these inadequate points, and the device tends to become large. On the other hand, as a relatively new method that is superior in terms of the ionization efficiency and the fragmentation, an atmospheric pressure dielectric barrier discharge method has begun to be studied in recent years. The barrier discharge ionizes the molecule in the sample by making a discharge current flow by applying a pulse-like or sine-wave-like high voltage via a dielectric barrier to a discharge unit in which the sample is introduced and that has a pressure close to the atmospheric pressure.
As a mass spectrometric device using the barrier discharge in an ionization unit, there are techniques described in PATENT LITERATURE 1 (JP 2012-104247 A), PATENT LITERATURE 2 (PCT/US2008/065245), PATENT LITERATURE 3 (PCT/JP2009/060653).
In PATENT LITERATURE 1, there is provided a small and lightweight mass spectrometric device capable of high precision mass spectrometry. The mass spectrometric device has an ionization source for ionizing a gas flowing in from the outside for ionizing a measurement sample, and a mass spectrometry unit for separating the ionized measurement sample. The barrier discharge is used for the ionization source. In PATENT LITERATURE 1, the mass spectrometric device has a suppression means for suppressing a flow rate of the gas taken into the ionization source, and an opening/closing means for opening and closing flow of the gas taken into the ionization source. By making the gas introduced from the outside intermittently flow into the ionization unit, and also by operating the barrier discharge unit intermittently at a pressure of 100 Pa to 10000 Pa lower than the atmospheric pressure, high efficiency and downsizing are achieved.
In PATENT LITERATURE 2, it is described about a method for obtaining high efficiency by ionizing the sample at the atmospheric pressure with the barrier discharge in the mass spectrometric device and discontinuously introducing the ionized sample to the mass spectrometry unit.
In PATENT LITERATURE 3, it is described about a method for improving ionization efficiency of the sample by devising electrode structure of the barrier discharge unit.
Although it is not an example of the barrier discharge, as a stabilization technique of the discharge unit, in relation to a device for detecting the discharge current, there are devices disclosed in PATENT LITERATURE 4 (JP 2011-232071 A), PATENT LITERATURE 5 (JP 2008-53020 A).
In PATENT LITERATURE 4, the device performs high S/N ionization current detection by detecting the discharge current of the discharge unit and integrating the ionization current in the device only in a period in which the discharge current flows.
In PATENT LITERATURE 5, it is described about a method for achieving noise reduction by detecting a current flowing through a discharge electrode and controlling an applied voltage so that the current becomes a predetermined current, to stabilize the ionization with the APCI (Atmospheric Pressure Chemical Ionization method) and reduce a noise level in the mass spectrometric device.